1. Field of the Invention
The present invention relates to an image forming system, and more particularly, to a method and apparatus for preventing a transfer roller from being contaminated by toner in an image forming system when the size of paper set in a printer driver is different from the size of paper supplied from a paper feeding cassette.
2. Description of the Related Art
In an electrophotographic image forming system, when an exposure unit radiates light onto a photosensitive medium charged to a predetermined potential, an electrostatic latent image is formed on the photosensitive medium. Thereafter, a developing unit forms a toner image by supplying toner to the electrostatic latent image. In general, a color electrophotographic image forming system needs four developing units in which four color toners, such as cyan (C), magenta (M), yellow (Y), and black (B), are respectively stored. The toner image is transferred onto paper directly from the photosensitive medium or via an intermediate transfer medium. When the transferred toner image passes a fusing unit, the toner image is fused on the paper by heat and pressure. A single color or multiple color image is printed on the paper through the above procedure.
As a basic rule for the above image forming system, paper having a size appropriate for the size of an image to be formed on the photosensitive medium is supplied according to printing conditions set in a printer driver. However, in some cases, paper of an appropriate size for the size of the image cannot be supplied. When the size of the image is smaller than the size of the paper, printing problems do not occur. However, when the size of the image is larger than the size of the paper, toner corresponding to portions from the toner image formed on the photosensitive medium is transferred to a transfer roller. As a result, next sheet of paper is contaminated, or a transfer bias potential of the transfer roller is substantially reduced, causing transfer defects.
In order to solve these problems, in conventional printers, the transfer roller is cleaned by changing a polarity used during an image transfer operation and transferring toner attached onto the transfer roller on the photosensitive medium before and after a printing operation is performed on new paper. However, this added step increases the amount of time required for driving the system. As such, the life span of the system may be reduced, and an entire time required for a printing operation may increase. Additionally, when the size of paper set in the printer driver is different from the size of paper supplied from a paper feeding cassette, jamming occurs and the printing operation is stopped. As such, additional efforts and time are needed to return to a printable state. In addition, since it is determined whether the size of the paper set in the printer driver is different from the size of the paper supplied from the paper feeding cassette after a first sheet of paper has been printed, when the size of an image is larger than the size of the paper, the transfer roller is contaminated by a toner image that has been previously formed on a transfer belt.